The present invention relates to method and apparatus for providing high-speed timing jitter correction or adaptive chromatic dispersion compensation for optical communication systems using a Return-To-Zero (RZ) modulation format.
Wavelength Division Multiplexing (WDM) has become a most effective technology for high capacity optical transmission systems. However, due to a finite optical bandwidth, it is desirable to be able to use more channels with smaller channel separations. In this regard, it was realized that a Return-To-Zero (RZ) modulation format has the benefit of higher spectral efficiency. In other words, it is possible to pack more channels into a given optical total bandwidth compared to a more conventional Non-Return-To-Zero (NRZ) modulation format. One of the major impairments for RZ or Soliton systems is the so-called xe2x80x9ctiming jitterxe2x80x9d, which term describes the fluctuations of the arrival time of the signal bits. A soliton is defined as a solitary wave that maintains its shape and velocity and does not widen or disperse in a normal way. There are several major causes for timing jitters in optical amplified transmission systems.
A first cause is the so-called Gordon-Haus jitter which is caused by optical amplifier noise as is described in the article entitled xe2x80x9cRandom walk of coherently amplified solitons in optical fiber transmissionxe2x80x9d by P. Gordon et al. in Optic Letters, Vol. 11, No. 10, at pages 665-667, 1986. A second cause is acoustic jitter which is caused by an acoustic wave generated by an optical signal as is described in the article entitled xe2x80x9cLong-range interaction of solitons in ultra-long communication systemsxe2x80x9d, by E. M. Dianov et al. in Soviet Lightwave Communications, Vol. 1, at pages 235-246, 1991. A third cause is the jitter that is caused by a nonlinear interaction among wavelength division multiplexed (WDM) channels. This third cause is usually a dominating factor among other timing jitters. A fourth cause is a soliton collision between adjacent bits of a single channel. This kind of nonlinear interaction is also found in both dispersion-managed soliton (DMS) systems and conventional Return-to Zero (RZ) systems. There are other sources of timing jitter such as jitter coming from transmitter electronics, and jitter transferred from electronic interfaces. The combined effect of nonlinear interaction between different wavelength channels and the random birefringence of transmission fibers also causes timing jitter.
An important characteristic of jitter is the frequency bandwidth of the jitter. It is possible to reduce the effect of timing jitter by improving a receiver""s design for situations in which the timing jitter happens in a time scale much longer than the bit period. In other words, the bandwidth of the jitter is much smaller than the bit rate. Many techniques have been used to cope with low bandwidth jitter, especially for conventional non-return-to-zero (NRZ) systems. In this regard see, for example, U.S. Pat. No. 5,608,757 (D. M. Smith et al.), issued on Mar. 4, 1997, U.S. Pat. No. 5,452,333 (B. Guo et al.) issued on Sep. 19, 1995, U.S. Pat. No. 5,425,060 (R. D. Roberts et al.) issued on Jun. 13, 1995, and U.S. Pat. No. 4,831,637 (V. B. Lawrence et al.) issued on May 16, 1989. The problem with these prior art techniques is that they do not work with the Return-To-Zero (RZ) format, especially for situations in which the jitter bandwidth is comparable to the bit rate.
Based on an observation that the timing jitter caused by soliton collisions is not random but is data pattern dependent, U.S. patent Ser. No. 5,710,649 (L. F. Mollenauer), issued on Jan. 20, 1998, describes a technique to cope with such non-random jitter. In Mollenauer, two parallel resonators are used to track variations in data patterns or timing jitter in a receiver. Although the tolerable jitter bandwidth was significantly improved compared to traditional methods mentioned hereinabove, the jitter bandwidth is still much smaller than the realistic jitter bandwidth in many applications. For example, in a 10 Gbit/sec. optical transmission system (e.g., OC192 SONET, or equivalent) using the RZ modulation format can have jitter components well beyond one GHz, while the Mollenauer method can only cope with a jitter component of about 200 MHz.
It is desirable to provide a technique for the transmission of digital optical signals, and more particularly, a technique for the transmission of digital optical signals, such as solitons or Return-To-Zero pulses, which compensates for timing jitter, polarization-mode dispersion, or third-order chromatic dispersion in single channel and multi-channel high speed optical transmission systems.
Viewed from one aspect, the present invention is directed to a compensation arrangement for receiving an optical input data signal comprising data pulses for each bit of data that have been subjected to a data signal change due to one of a group consisting of timing jitter and chromatic dispersion. The compensation arrangement comprises a clock recovery arrangement, a phase modulator, and a dispersive unit. The clock recovery arrangement generates an electrical clock output control signal including a predetermined phase modulation depth and phase and a data rate of the received optical input data signal. The phase modulator is responsive to the optical input data signal received by the compensation arrangement and the electrical clock output control signal from the clock recovery arrangement for generating an optical output signal wherein the phase of the data signal change associated with each data bit is delayed by a predetermined amount. The dispersive unit introduces a predetermined amount. The dispersive unit introduces a predetermined amount of dispersion to the pulse in the optical output signal from the phase modulator for generating an output optical signal from the compensation arrangement wherein the data signal change for each data bit are in phase.
Viewed from another aspect, the present invention is directed to a compensation arrangement for receiving an optical input data signal comprising a data pulse for each bit of data that have been subjected to a data signal change due to one of a group consisting of timing jitter and chromatic dispersion. The compensation arrangement comprises a clock recovery arrangement, a phase modulator, and a dispersive unit. The clock recovery arrangement receives the optical input data signal and generates therefrom an electrical clock output control signal comprising both a frequency corresponding to a bit data rate of the optical input data signal and a predetermined phase modulation depth and phase. The phase modulator receives the optical input data signal and modulates the phase thereof in accordance with the electrical clock output control signal from the clock recovery arrangement including a phase difference of 90 degrees from that of the optical input data signal. The phase modulator generates an optical output signal wherein the phase of the data signal change component associated with each data bit is delayed by a predetermined amount. The dispersive unit introduces a predetermined amount of dispersion to the data pulses in the optical output signal from the phase modulator for generating an output optical signal from the compensation arrangement wherein the data pulses for each data bit are in phase with clock pulses generated in the clock recovery arrangement.
Viewed from still another aspect, the present invention is directed to a method of providing compensation to each bit of data that has been subjected to data signal changes due to one of a group consisting of timing jitter and chromatic dispersion in data signals propagating in an optical transmission system comprising the following steps. In a first step, an optical signal is received including data pulses that have been subjected to a data signal changes. In a second step, an electrical clock output control signal is generated comprising both a frequency corresponding to a bit data rate of the optical input data signal and a predetermined phase modulation depth and phase in a clock recovery arrangement from the received optical input data signal. In a third step, the phase of the optical input data signal received in step (a) is modulated in a phase modulator in accordance with the electrical clock output control signal generated in the first step wherein said phase is a phase difference of 90 degrees from that of the optical input data signal. Still further, the phase modulator generates an optical output signal wherein the phase of the data signal change component associated with each data bit is delayed by a predetermined amount. In the fourth step, a predetermined amount of dispersion is introduced to the data pulses in the optical output signal from the phase modulator by a dispersive unit for generating an output optical signal from the compensation arrangement wherein the data signal change for each data bit is compensated for.